Single coil absolute position sensors that are used to sense the position of an object require an excitation signal that is of the form of a constant frequency sinusoidal wave. The output of the sensor is an amplitude modulated signal, modulated by the position of the object. The single coil absolute position sensor usually includes an excitation coil, and a sensor component. The sensor component is typically affixed to the object that is being sensed, whereas the excitation coil is not affixed to the object that is being sensed. Additionally, the excitation coil and the object being sensed are free to move relative to one another. In some aspects, the position of the excitation coil is fixed while the object being sensed and the affixed sensor component are free to move in a linear motion or in an angular motion. In other aspects, the position of the object being sensed and the affixed sensor component is fixed whereas the excitation coil is free to move in a linear motion or angular motion.
The function of the excitation coil is to transmit the excitation signal to the sensor component. The sensor component receives the excitation signal and uses this received excitation signal to output an amplitude modulated signal, modulated by the position of the object being sensed.
The amplitude modulated signal that is output by the single coil absolute position sensor may be demodulated to recover the position of the object being sensed. Conventional demodulator circuits may perform this demodulation of the amplitude modulated signal, but their performance is typically very sensitive to any variations in the amplitude of the excitation signal. Such variations in the amplitude of the excitation signal are quite common, and may result from degradation in the signal due to the air gap between the source of the excitation signal and the demodulator circuit.
Conventional demodulator circuits are usually composed of many individual components which each act on the excitation signal and the amplitude modulated signal. These extensive stages of electronics induce more noise and error in the demodulated signal. Additionally, conventional demodulator circuits yield a demodulated signal that contains ripples.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a demodulator apparatus that is insensitive to variations in the amplitude of the excitation signal, does not induce significant noise in the demodulated signal due to extensive stages of processing of the signals, and do not include ripples in the demodulated output signal. There is also a need for improved methods of accurately sensing the position of an object.